In recent years, with enhancement of operation speed of personal computers, widespread use of network infrastructure, and realization of mass storage of data, as well as cost reduction of data storage, there are increasing occasions in which information of documents and images, having been conventionally provided in the form of printed paper matter, is received and viewed as more convenient electronic information.
As viewing methods for such electronic information, there are mainly used those which are of light emitting types such as conventional liquid crystal display devices and CRTs, or organic EL display devices, which have recently been marketed. Especially, however, when electronic information is composed of items of document information, it is necessary to stare at these viewing devices for a relatively long time, which is certainly not viewer-friendly. It is commonly known that light emitting type display devices have disadvantages such as eye fatigue due to flicker, inconvenience of portability, limited reading posture, necessity to look closely at still images, and high power consumption due to the use of these devices for long-time reading.
As display devices to overcome these disadvantages, there are known memory effect reflective display devices, which utilize external light, resulting in consuming no electrical power to retain images. However, these devices do not exhibit adequate performance due to the following reasons.
Namely, a system, employing a polarizing plate such as a reflective type liquid crystal, creates a problem in white display due to its low reflectance of approximately 40%, and most of the production methods of constituent members are neither simple nor easy. Further, polymer dispersion type liquid crystals require a high operating voltage and exhibit poor contrast of resultant images due to the utilization of a refractive index difference between the used organic compounds. Still further, polymer network type liquid crystals have problems such that high operating voltages result and complicated TFT circuits are required to enhance memory capability. Yet further, display elements employing electrophoresis require a high operating voltage of at least 10 V and tend to exhibit low operation life due to electrophoretic particle aggregation. In contrast, although being drivable at a low voltage of at most 3 V, electrochromic display elements have the disadvantage of exhibiting poor color quality of black or colors (such as yellow, magenta, cyan, blue, green, and red), as well as having the disadvantage that each display cell requires a complicated film structure such as a vapor deposition film to ensure memory capability.
As a display system to overcome any of the disadvantages in each of the systems described above, there is known an electrode deposition (hereinafter referred to simply as ED) system employing dissolution and deposition of a metal or metallic salt. The ED system is drivable at a low voltage of at most 3 V, and features advantages such as a simple cell structure and excellence in black and white contract, as well as in black image quality, resulting in disclosure of a variety of systems (for example, refer to Patent Documents 1-3).
The present inventor has made detailed investigation in the technologies disclosed in each of the Patent Documents described above, and found that these conventional technologies have problems of exhibiting color change of white color during white display after accumulated repeating driving operation. In addition, it is known that an onium cation is applied for an electrolytic capacitor (for example, refer to Patent Documents 4-5). However, its application to a display element was not described or suggested.
Patent Document 1: U.S. Pat. No. 4,240,716 specification
Patent Document 2: Japanese Patent Publication No. 3428603
Patent Document 3: Unexamined Japanese Patent Application Publication No. 2003-241227
Patent Document 4: Japanese Patent Publication No. 2701874
Patent Document 5: Unexamined Japanese Patent Application Publication No. 2005-5357